The present invention relates to the production of composite structures implemented for applications in the fields of microelectronics, optics and/or optoelectronics. More precisely, the invention relates to a general method of forming a structure comprising a layer in a semiconductor material taken from a donor substrate, such a method typically includes the following successive steps:                implantation of atomic species to form an embrittlement zone in a donor substrate at a given depth;        assembly of the donor substrate to a receiver substrate;        supply of energy to detach the layer taken from the donor substrate at the embrittlement zone; and        finishing treatment for the layer removed to improve its surface condition.        
The types of methods mentioned above are already known to the person skilled in the art. SMART-CUT® methods are one example that corresponds to a preferred embodiment of the invention. Such methods allow structures having a thin layer in a semiconductor material to be produced. In order to obtain specifics on the abovementioned technology, the person skilled in the art may, for example, refer to by G. Celler, Frontiers of Silicon-on-Insulator, Journal of Applied Physics, Vol 93, no. 9, May 1, 2003, pages 4955-4978. The structures presently described are of the Silicon On Insulator (SOI) type, in which the layer taken is in silicon, or of the strained SOI (sSOI) type, in which the layer taken is in strained silicon. Other types of composite structures may also be obtained.
Further concerning implanting the general method for fabricating SOI or similar structures, the implantation step includes implanting one or more ionized species in the donor substrate which subsequently will form a zone of defects that are more or less buried within the substrate at a depth that at least partially depends on the implantation energy. These defects will be able to develop and will be used in the detachment step. The energy utilized in the detachment step is at least partly supplied in thermal form by a process referred to herein as detachment annealing. During heat annealing, the moment when the layer to be removed form the donor substrate detaches will depend on both the temperature at which the process is carried out and the duration of the exposure to the temperature. This pair of factors is known as the “heat budget.” Beyond the heat budget, temperature distribution (between the top and the bottom of the oven) is also important.
After the detachment step, roughness, imperfections, or reduction in the crystalline quality of the separation surface of the removed layer may generally be observed. Given the specified applications for these substrates, the requirements for the surface condition of the structures utilized are generally very strict: the roughness of the thin layer is a parameter that to a certain extent determines the quality of the components that will be made on the structure. To treat these surface defects, a finishing treatment may be implemented with the object of conforming to the final roughness requirements that the free face of the substrate must meet for its subsequent use. These finishing steps correspond to additional steps of the method which tend to make the method more complex and more costly.
A known method to reduce surface defects such as those mentioned above consists of carrying out detachment annealing at a “high temperature,” which usually corresponds to a temperature of over 500° C. US Patent Application Pub. No. 2003/0216008 and International Patent Application Pub. No. WO 2005/086228 provide examples of such detachment annealings, particularly by exposing the wafers to a high temperature for a given duration in order to initiate detachment. These references demonstrate that roughness is reduced when detachment annealing is performed in part at a high temperature, which consequently allows the finishing step to be simplified. In fact, it is thought that the act of prolonging detachment annealing at a high temperature allows certain surface defects issued from the detachment to be “healed.” However, the act of carrying out detachment annealing at such temperatures produces an undesirable effect in that it is sometimes difficult to detach the donor substrate from the structure produced. This effect is generally explained by a reattachment phenomenon of the donor substrate and of the structure produced at the detachment interface when the assembly is subjected to a high temperature.